I'm not a veterinarian, but being in the veterinary field for 15 years, I've seen some things. :) With Pit Bulls especially, I've noticed that you can certainly tell when you have an un-neutered male. Their muscular stature is more developed than in a neutered male. Same thing goes for cats. Male cats that aren't neutered develop a larger head due to the testosterone. Not sure what the vets would say about it, but I've seen the difference.